1. Field of the Invention
The invention relates to a fixing apparatus which is used for a printer, a copying apparatus, or the like to which an electrophotographing method is applied.
2. Related Background Art
Hitherto, a fixing apparatus is constructed by a fixing roller having a heat source therein and a pressing roller which is come into pressure contact with the fixing roller. FIG. 12 shows the conventional fixing apparatus. In FIG. 12, a fixing apparatus 1 is used for a monochromatic printer and has a fixing roller 2 and a pressing roller 3. The fixing roller 2 is constructed in such a manner that a fluorine mold releasing layer 5 for releasing toner is formed on a surface of a metal pipe 4 such as aluminum or iron having excellent thermal conductivity, and a heat source 6 is provided in the fixing roller. The pressing roller 3 is come into pressure contact with the fixing roller 2 by a pressing spring 7 and is rotated in association with the rotation of the fixing roller 2. A core shaft 9 is coated with a foam rubber layer 8 having excellent heat insulating performance, thereby forming the pressing roller 3.
When the pressing roller 3 is come into pressure contact with the fixing roller 2, a nip layer 10 is formed. When a print medium 12 which holds unfixed toner 11 passes through the nip layer 10, the unfixed toner 11 is fixed onto the print medium 12. Since the nip layer 10 is in such a shape that a portion on the side of the pressing roller 3 is dented, after the print medium 12 nipped by the nip layer 10 goes out of the nip layer 10, it is conveyed along an outer peripheral surface of the fixing roller 2 and can be easily wrapped around the fixing roller 2. Therefore, a separating nail 13 is provided on the surface of the fixing roller 2 in a contact state, thereby peeling off the print medium 12 from the fixing roller 2.
However, if the separating nail is in contact with the surface of the fixing roller, the surface of the fixing roller is damaged and an adverse influence is exerted on picture quality. To avoid such a problem, fundamentally, there is a case where no separating nail is provided in the color printer in which high picture quality is required in particular.
A fixing apparatus having no separating nail has been disclosed in, for example, JP-A-2000-56601. According to the technique disclosed in this Official Gazette, hardness of the pressing roller is set to be higher than that of the fixing roller, thereby forming such a nip shape that the pressing roller bites the fixing roller, so that a print medium is curved toward the pressing roller side.
However, in the above conventional apparatus, if the hardness of the fixing roller is reduced and the hardness of the pressing roller is simply increased, the nip width inevitably becomes narrow and an amount of heat necessary for fixing cannot be supplied to the toner and the sufficient pressure necessary for fixing cannot be obtained. Particularly, in the fixing apparatus which is used for the color printer, since a high pressure is necessary and a toner portion of a single layer and a toner portion of multilayers exist, the pressure cannot be uniformly applied to those toner portions. If the hardness of the pressing roller made of a foam material is increased, physical properties of the pressing roller have to be set to physical properties close to those of a solid rubber by further reducing the expansion ratio of the foam material, so that high heat insulating performance cannot be obtained.